To model stiffness in athletes, researchers or practitioners will commonly seek to determine vertical stiffness during bilateral hopping.

Vertical stiffness is the quickest and easiest stiffness measure to determine as all you need is a force plate. Vertical ground reaction force is your force measure and then displacement of the centre of mass can be determined from the force trace using inverse dynamics.

Bilateral hopping is a popular choice of assessment because it provides the closest representation of a true spring mass-model; hopping is an efficient activity that requires little active force contribution once a rhythm is established.

Problems with stiffness assessments

The problem with assessing stiffness, or any pretty much any variable within biomechanics, is that it’s almost impossible to truly assess a ‘mechanical’ property during human movement. Every movement task is a skill and is therefore governed by a host of conscious and sub-conscious control mechanisms.

Stiffness is therefore dependent on: a) the movement task, b) how we perform the task, and c) our training background.

We discovered that significant asymmetries between limbs were exhibited in each type of task, however, the way in which these asymmetries presented was different in each task.

Relative to the stiffer limb, vertical stiffness of the compliant (less stiff) limb was 5% less in bilateral hopping, 20% less during bilateral drop jumping and 15% during unilateral drop jumping.

These findings led us to conclude that all three methods could be used to assess vertical stiffness asymmetries but drop jumping tasks detected larger asymmetries than hopping.

Key Point 1

The magnitude of asymmetries is likely to be greater in maximal tasks such as drop jumps versus rhythmic tasks such as hopping.

Our findings – direction of asymmetry

We also discovered that the stiffer leg was rarely consistent across all three tasks. For example, an individual may present a stiffer right leg during bilateral hopping, left leg during bilateral drop jumping and right leg during unilateral drop jumping.

Only four participants in the study had the same leg characterised as the stiffer leg in all three tasks.

Key Point 2

The presentation and direction of asymmetries is likely to be task dependant.

What’s the best way to assess stiffness it athletes?

In short, it depends on the athlete and what they do. Much like choosing an agility test, we should seek to find the task with the closest correspondence to an athlete’s sport.

For example, a bilateral hopping task may be well suited to a distance runner engaging in prolonged submaximal activity. Conversely, a unilateral drop jump is likely to be a more valid test for assessing stiffness in team-sport athletes where maximal bursts of unilateral activity, such as changes of direction, are required.